The present invention relates to charge splitting devices and more particularly to a Charge Coupled Device (CCD) structure in which an incoming charge packet is split into multiple outgoing charge packets as a result of a charge gradient.
Charge Coupled Devices (CCDs) provide a basic function of storing and moving isolated packets of charge. Various operations can be performed on the charge packets. For example, they can be added (merged), split into two or more pieces, conditionally steered, destructively or nondestructively sensed, and the like. These operations make it possible to use CCD based circuits for various discrete time analog signal processing operations, by having signals represented as charge packets.
In the following descriptions, the use of “4-phase” CCD technology, with two general types of gates, is assumed. These two types of gates are so-called “storage gates” and so-called “barrier gates.” Storage gates are gates under which charge packets reside during appreciable periods of time. Barrier gates are gates under which charges pass dynamically but are not generally stored. Storage and barrier gates may be constructed in two separate layers of gate material, and can overlap. Alternatively, storage and barrier gates may be constructed in a single layer of gate material without overlap.
A charge splitter is one structure that can be built from storage gates and barrier gates. In a charge splitter, a single incoming charge packet is divided into two outgoing packets. The splitting ratio, that is the ratio of the charge of the two outgoing packets, is typically a fixed design parameter of the structure.
One type of non-adjustable charge splitter uses storage and barrier gates arranged in series. The input charge to be split is first fed to a special type of storage gate, called a “splitting gate” herein. The splitting gate provides a structure in which the incoming charge packet is temporarily stored. The channel underneath the splitting gate is physically divided into two sections at an output portion. Thus, when the stored charge is allowed exit the splitting gate, as the charges spill over one or more outgoing barrier gates, the separation of charges is maintained. Each separated charge is then collected and stored in a separate output storage gate.
With this design, the ratio of the split is fixed by the geometry of the channel underneath the splitting gate. The splitting process depends upon both the initial distribution of charge under the splitting gate, and the charge outflow rate from the splitting gate to the respective output storage gates.
In this approach, the splitting operation occurs dynamically, in the sense that the split occurs when charge is actively moved from one storage gate to another. However, the intended amount of the split is fixed and determined in advance.
Unfortunately, although the splitting ratio is intended to be fixed, it can be subject to variations in implementation. These variations occur for multiple reasons, but may be due to Integrated Circuit (IC) process variations (such as differences in photo masking processes, gate threshold levels and the like) as well as operating conditions (such as supply voltage, temperature, external noise sources, and the like). In this case, even when the desired split is a fixed ratio, such a circuit allows for the use of feedback techniques to obtain a more precise result under a variety of continuously varying operating conditions.
In other instances, it would be desirable to provide for an adjustable splitting ratio, that can be determined while the circuit is operating. This would not only permit correction of a fixed split ratio for process variations, but could also be used to provide a generalized circuit function of splitting a charge based on a variable ratio determined by the value of another input signal.